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Abstract

We present the design of a novel collector mirror for laser produced plasma (LPP) light sources to be used in extreme ultraviolet (EUV) lithography. The design prevents undesired infrared (IR) drive laser light, reflected from the plasma, from reaching the exit of the light source. This results in a strong purification of the EUV light, while the reflected IR light becomes refocused into the plasma for enhancing the IR-to-EUV conversion. The dual advantage of EUV purification and conversion enhancement is achieved by incorporating an IR Fresnel zone plate pattern into the EUV reflective multilayer coating of the collector mirror. Calculations using Fresnel-Kirchhoff’s diffraction theory for a typical collector design show that the IR light at the EUV exit is suppressed by four orders of magnitude. Simultaneously, 37% of the reflected IR light is refocused back the plasma.

Figures (5)

Schematic drawing of (a) a typical EUV source based on laser produce plasma (LPP). The reflected IR and generated EUV light both follow the same path through the exit (red-blue area). (b) The same source using a Fresnel zone plate on the collector mirror. Only the EUV passes through the exit aperture (blue), while reflected IR (red) is refocused into the plasma.

Decrease of the focus diameter (FWHM) generated by the Fresnel zone pattern in the plasma plane versus the number of contributing Fresnel zones. Currently used plasma sources have a diameter between 300 and 100 μm which is indicated by the light and dark shaded areas.

Intensity distribution at the intermediate focal plane of the collector which is also the exit of the source (a) with and (b) without the Fresnel zone plate on the collector. Exit apertures with 900 μm diameter are shown in the zoomed insets.